SIDE EFFECTS: Dizziness, drowsiness, weakness, blurred vision, dry eyes, dry mouth, nausea, constipation, and abdominal bloating may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. To relieve dry mouth, suck (sugarless) hard candy or ice chips, chew (sugarless) gum, drink water, or use a saliva substitute. To relieve dry eyes, consult your pharmacist for artificial tears or other eye lubricants. To prevent constipation, eat a diet adequate in fiber, drink plenty of water, and exercise. Consult your pharmacist for help in selecting a laxative (such as a stimulant type with stool softener). Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Tell your doctor right away if you have any serious side effects, including: decreased sweating, dry/hot/flushed skin, fast/irregular heartbeat, loss of coordination, slurred speech, fainting, mental/mood changes (such as confusion, agitation, unusual excitement, depression, rare thoughts of suicide), difficulty urinating, decreased sexual ability, signs of anemia (such as unusual tiredness, pale skin), signs of infection (such as fever, persistent sore throat), easy bruising/bleeding. Get medical help right away if you have any very serious side effects, including: eye pain/swelling/redness, vision changes (such as seeing rainbows around lights at night), slow/shallow breathing. A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

PRECAUTIONS: Before taking this medication, tell your doctor or pharmacist if you are allergic to hyoscyamine, atropine, scopolamine, or phenobarbital; or to primidone; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details. Before using this medication, tell your doctor or pharmacist your medical history, especially of: personal or family history of glaucoma (angle-closure type), problems with urination (due to enlarged prostate, a blocked urinary tract), other stomach/intestinal problems (such as slow gut, blockage, ulcerative colitis, infection, little/no stomach acid, ileostomy/colostomy patients with diarrhea), overactive thyroid, heart problems (such as coronary artery disease, angina, congestive heart failure, fast/irregular heartbeat, heart problems due to severe bleeding), high blood pressure, heartburn problems (such as acid reflux, hiatal hernia, esophagus problems), certain nervous system problem (autonomic neuropathy), myasthenia gravis, liver problems, kidney problems, a certain blood disorder (porphyria), restlessness or excitement during past use of phenobarbital, breathing/lung problems, personal or family history of a substance use disorder (such as overuse of or addiction to drugs/alcohol), mental/mood conditions (such as depression, thoughts of suicide, psychosis). This drug may make you dizzy or drowsy or blur your vision. Alcohol or marijuana can make you more dizzy or drowsy. Do not drive, use machinery, or do anything that needs alertness or clear vision until you can do it safely. Avoid alcoholic beverages. Talk to your doctor if you are using marijuana. The liquid form of this product may contain alcohol or sugar. Caution is advised if you have diabetes, alcohol dependence, liver disease, or any other condition that requires you to limit/avoid these substances in your diet. Ask your doctor or pharmacist about using this product safely. This medication may make you sweat less, making you more likely to get heat stroke. Avoid doing things that may cause you to overheat, such as hard work or exercise in hot weather, or using hot tubs. When the weather is hot, drink a lot of fluids and dress lightly. If you overheat, quickly look for a place to cool down and rest. Get medical help right away if you have a fever that does not go away, mental/mood changes, headache, or dizziness. Older adults may be more sensitive to the side effects of this drug, especially drowsiness, confusion, unusual excitement, constipation, and urination problems. Drowsiness and confusion can increase the risk of falling. Children may be more sensitive to the side effects of this drug. This drug can often cause excitement instead of drowsiness in young children. This medication is not recommended for use during pregnancy. It may harm an unborn baby. Consult your doctor for details. Hormonal birth control products (such as pills, patch, ring) may not be effective if taken with this medication (see also Drug Interactions section). Discuss reliable forms of birth control with your doctor. This medication passes into breast milk and may have undesirable effects on a nursing infant. Consult your doctor before breast-feeding.

DRUG INTERACTIONS: See also the How to Use section. Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval. Some products that may interact with this drug include: darunavir, etravirine, potassium tablets/capsules, rilpivirine, sodium oxybate, drugs that are affected by slowed gut movement (such as pramlintide). This medication may affect the absorption of other products such as griseofulvin, certain azole anti-fungal drugs (ketoconazole, itraconazole), slowly dissolving forms of digoxin, among others. Other medications can affect the removal of phenobarbital from your body, which may affect how this product works. Examples include St. John's wort, among others. Phenobarbital can speed up the removal of other medications from your body, which may affect how they work. Examples of affected drugs include artemether/lumefantrine, asunaprevir, atazanavir, boceprevir, cobicistat, lurasidone, ranolazine, sofosbuvir, sorafenib, voriconazole, certain calcium channel blockers such as felodipine/nimodipine, among others. This medication may decrease the effectiveness of hormonal birth control such as pills, patch, or ring. This could cause pregnancy. Discuss with your doctor or pharmacist if you should use reliable backup birth control methods while using this medication. Also tell your doctor if you have any new spotting or breakthrough bleeding, because these may be signs that your birth control is not working well. The risk of serious side effects (such as slow/shallow breathing, severe drowsiness/dizziness) may be increased if this medication is taken with other products that may also cause drowsiness or breathing problems. Tell your doctor or pharmacist if you are taking other products such as opioid pain or cough relievers (such as codeine, hydrocodone), alcohol, marijuana, other drugs for sleep or anxiety (such as alprazolam, lorazepam, zolpidem), muscle relaxants (such as carisoprodol, cyclobenzaprine), or antihistamines (such as cetirizine, diphenhydramine). Check the labels on all your medicines (such as allergy or cough-and-cold products, diet aids) because they may contain ingredients that cause drowsiness or a fast heartbeat. Ask your pharmacist about using those products safely. This medication may interfere with certain laboratory tests (including gastric secretion tests), possibly causing false test results. Make sure laboratory personnel and all your doctors know you use this drug. Phenobarbital is very similar to primidone. Do not use medications containing primidone while using phenobarbital.

OVERDOSE: If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call their local poison control center at 1-800-222-1222. Canada residents can call a provincial poison control center. Symptoms of overdose may include: large pupils, hot/dry skin, fever, severe dizziness, severe drowsiness, severe thirst, mental/mood changes (such as confusion, unusual excitement), fast/irregular heartbeat, muscle weakness, inability to move (paralysis), slowed breathing, fainting, inability to wake up, seizures.